There is a plurality of fields of application for sensors. These include the use of gas sensors for early fire detection. Thus, prior German patent application 197 41 335.8 describes optical gas sensors which are based on an interaction of specific gases with a semitranslucent layer, an absorption factor of light of a specific wavelength being dependent on the gas concentration. Disadvantageous in the known optical gas sensors are the relatively complex and voluminous measurement set-ups since, apart from an optical transmitter and an optical receiver, it is required to arrange a gas-sensitive layer within a beam path between these two components. In particular, measurements where the intention is for gas concentrations to be measured at points which are spatially distant from the optical components and which are, for example heavily thermally stressed, are possible only with difficulty.
The optical sensor according to the present invention offers the advantage that very compact and inexpensive, integrated components are picturable because of the spatial separability of the at least one optical transmitter and the at least one optical receiver and because of a sensitive layer which interacts with a sample, for example, a gas or a gas mixture, and which changes the transmission of light of a specific wavelength. If an integrated module preferably composed of an optical transmitter and of an optical receiver, is coupled via optical waveguides to the sensitive layer, which can be installed at any distant location, these units can completely be spatially separated from one another and, consequently, the gas-sensitive layer can be positioned at locations where, due to the spatial conditions or because of the thermal or mechanical conditions, no sensitive optical and/or electronic components can be installed.
By using, as the sensitive element, a gas-sensitive layer or membrane, also designated as optode, which is substantially permeable to electromagnetic radiation, and which changes its absorption properties and/or its refractive index for electromagnetic radiation in response to a contact with a gas or a gas mixture, very compact and miniaturizable gas sensors can be manufactured in simple manner. Understood by an optode in connection with the present invention, are, in particular, polymer layers which, due to indicator substances intercalated therein, exhibit a dependency of the light transmission on the concentration of a specific gas in the atmosphere surrounding the optode. Optodes which are used according to the present invention respond to the concentration of a specific gas in a selective and reversible manner. By measuring the absorption properties of the indicator substance which is present in the gas-sensitive layer or membrane, and which is exposed to and interacts with the gas, it is possible for very low gas concentrations to be measured and detected using relatively simple optical devices. The indicator substance present in the gas-sensitive layer and preferably intercalated in a polymer matrix, preferably responds only to a specific gas so that, using different indicator substances, sensors acting in a gas-specific manner are picturable, respectively.
In an advantageous embodiment of the optical sensor, a gas-sensitive layer having an applied or integrated indicator substance is interpositioned in the beam path of at least one source for electromagnetic radiation, preferably an optical transmitter, and of at least one detector for electromagnetic radiation, preferably an optical receiver, the gas-sensitive layer changing the transmission or absorption properties for the electromagnetic radiation depending on the physical and/or chemical interaction with a specific gas. The gas-sensitive layer is coupled to the transmitter and to the receiver via at least one optical waveguide. The source for electromagnetic radiation can be, for example, a light-emitting diode as optical transmitter, the light-emitting diode emitting light having a suitable wavelength. Accordingly, a photodiode is possible as optical receiver having a frequency range matched to the emitted wavelength of the light-emitting diode. Such a design can be implemented in simple manner with very inexpensive component parts. The gas-sensitive layer with the indicator substance contained therein or applied thereto, which is arranged in the beam path between optical transmitter and optical receiver is quantitatively calibrated at specific light wavelengths, preferably in a manner corresponding to its absorption properties, so that different gases can be detected using different light wavelengths with differently responding indicator substances.
Furthermore, it is advantageous to combine a plurality of optical transmitters with a plurality of gas-sensitive layers capable of detecting different gases, respectively, so that different gases can be detected using a single device. It can also be advantageous for the at least one optical transmitter and/or the at least one optical receiver to be directly provided or coated with a gas-sensitive layer. In this manner, a plurality of optical receivers provided with layers which are sensitive to different gases, respectively, can be irradiated by a plurality of optical transmitters or also by only one optical transmitter which, in this case, covers the entire required wavelength range.
The beam path between optical transmitter and optical receiver, including the intermediate gas-sensitive layer, can be extended in advantageous manner by using optical waveguides as optical coupling elements. In this manner, at least two optical transmitters and receivers positioned at different locations can be optically coupled with a gas-sensitive layer. Also possible is a spatial assembly of optical transmitter and optical receiver at the same location as well as an optical coupling over a very large distance using two optical waveguides led in parallel. The optical waveguides can be diverted or bent at their coupling point to the gas-sensitive layer in such a manner that their end faces, at which the light emerges or enters perpendicularly, are located in a parallel manner opposite each other, the gas-sensitive layer lying in between in the optical axis.
In an advantageous embodiment, the ends of the optical waveguides to be optically coupled to the gas-sensitive layer are chamfered at their sides facing away from the gas-sensitive layer in such a manner that the light beam guided in the optical waveguides is reflected toward the air at these interfaces (boundary surfaces). If the ends of the optical waveguides are chamfered to 45xc2x0, the light is reflected by 90xc2x0 correspondingly, and exits the respective optical waveguide perpendicularly to the longitudinal direction thereof. If one end of the optical waveguide or both ends are coated with a gas-sensitive layer in a manner that the gas-sensitive layer is located in the beam path of a light beam running between the optical waveguides in the medium to be examined, it is possible for an optical detecting device for gases which has an extremely compact and simple design to be implemented at an arbitrary mounting location.
In a further advantageous embodiment, provision is made to use only one optical waveguide which, due to its special design including a conical peak and a gas-sensitive layer applied thereto, allows gases to be effectively detected also at locations which can be very far away from optical transmitter and receiver. In this context, at one end of the optical waveguide, the optical transmitters and receivers are arranged in such a manner that the light beam emanating from the optical transmitter runs parallel to the light beam guided toward the optical receiver within the optical waveguide. Therefore, the optical waveguide used in this connection expediently has a diameter which allows optical transmitters and receivers which can be considerably miniaturized to be arranged side by side. At the other end, the optical waveguide expediently has a conical peak which is partially or completely covered with a gas-sensitive layer. In this context, the cone angle should approximately be 90xc2x0, to achieve a desired twice repeated deflection of the light by 90xc2x0, i.e., by a total of 180xc2x0. Since, due to nearly identical refractive indices of the optical waveguide and of the coating, the light can penetrate the interface between the optical waveguide and the coating in nearly unhindered manner, i.e., without reflection, but is reflected at the interface of the coating toward the surrounding medium, for example, air because of the markedly different refractive indices, a twice repeated reflection by 90xc2x0 takes place. The light beam does not exit the measurement set-up but runs back inside the optical waveguide toward the optical receiver. In such an arrangement, the gas-sensitive layer is consequently crossed by a light beam on four paths, respectively, as a result of which changes in the absorption properties due to a detected gas has a greater effect on the signal registered by the optical receiver than in the case that a gas-sensitive layer is penetrated only once or twice. Apart from a structural simplification due to only one optical waveguide, it is possible for such arrangements to be miniaturized particularly well.
In a preferred application, optical sensors of that kind can be used for analyzing and/or monitoring exhaust gases of internal combustion engines, in particular of internal combustion engines of motor vehicles. In this manner, the measured values obtained from the optical sensor and from an evaluation unit arranged downstream can be used for controlling the internal combustion engine, for example, for complying with exact exhaust values and/or for monitoring and controlling a catalytic afterburning in a catalytic converter in the exhaust train of the internal combustion engine. It is also possible to accurately determine the stoichiometric air ratio, the so-called xe2x80x9clambda valuexe2x80x9d, in exhaust gases of internal combustion engines by determining the contents of O2, CO, CO2, HC, and NOx. By using a plurality of such sensors, it is also possible to carry out a so-called xe2x80x9cexhaust-optimizedxe2x80x9d engine management as well as a permanent monitoring of the combustion processes, as a result of which both the fuel consumption of the internal combustion engine and its exhaust behavior as well as its wear behavior can be optimized.
Moreover, the device according to the present invention can be advantageously used for monitoring an air quality, for example, for controlling ventilation dampers in air conditioning systems. Likewise, devices according to the present invention can be advantageously used for ventilation control and air conditioning in interior spaces. Furthermore, optical sensors according to the present invention can be used for monitoring and/or controlling combustion plants, furnaces, or power plants operated with hydrocarbons. A further possible use is, for example, the measurement of NH3-concentrations in cold storage houses and refrigeration plants using ammonia as a cooling agent. Of course, such optical sensors are also suited for smoke and/or fire detectors, the determination of fire-reference gases by individual optical sensors or a combination of a plurality of sensors allowing the detection and alarm times to be strongly reduced compared to known devices, and allowing the safety against false alarms to be significantly increased.